Minimization of drive tests (MDT) is a means to compensate or partially replace the costly drive tests a network (NW) operator will otherwise have to perform by configuring a selection of user equipments (UEs) in Active/Idle modes to do certain types of measurements, such as those described in 3GPP Technical Report (TR) 36.805 v9.0.0, Study on Minimization of Drive-Tests in Next Generation Networks (Release 9) (December 2009). A UE can be selected based on its International Mobile Subscriber Identity (IMSI), International Mobile Equipment Identity (IMEI), area, device capabilities, and any combination thereof. The functionality is being standardized for LTE (Long-Term Evolution) and UMTS (Universal Mobile Telecommunications System) networks.
Use cases for MDT include coverage optimization, mobility optimization, capacity optimization, parameterization for common channels, and Quality of Service (QoS) verification. For coverage optimization by MDT, a UE can periodically measure downlink pilot signal levels (e.g., Common Pilot Channel (CPICH) Received Signal Code Power (RSCP), Common Pilot Channel Energy per chip to Noise ratio (CPICH Ec/No), or Time Division Duplexing (TDD) Primary Common Control Physical Channel (P-CCPCH) Received Signal Code Power (RSCP) and Interference Signal Code Power (ISCP), Reference Signal Received Power (RSRP) and Reference Signal Received Quality (RSRQ)), determine whether its serving cell signal level has become worse than a threshold, determine whether its transmit power headroom is less than a threshold, determine whether it has a Paging Channel failure (e.g., a Paging Control Channel (PCCH) decode error) or a Broadcast Channel failure.
Two modes of MDT are currently considered: immediate MDT and logged, or deferred, MDT. In immediate MDT, a UE in a Connected-mode state performs measurements and immediately reports those measurements to an evolved NodeB (eNodeB, or eNB) in an LTE network or a radio network controller (RNC) in a UTRAN that is available at the time of the report. In logged MDT, a UE in Idle mode performs measurements when predetermined conditions are satisfied and stores the measurements in a log for reporting to a radio node such as eNodeB in LTE and RNC in UMTS at a later point in time. The stored measurements are reported either periodically or upon a trigger, and the measurements can be single-shot (one-time) measurements or an average of measurements over a specified period. Logging and reporting intervals are typically on the order of seconds.
A UE has either zero or one radio resource control (RRC) connection to an eNB in an LTE network or to a base station (BS) and RNC in a UTRAN. A UE having an active communication with the network is typically in a Connected-mode state, whilst UEs not having transmissions to the network may be in the IDLE mode. In an LTE network, the only Connected-mode state currently specified is denoted RRC_CONNECTED, and in a UTRAN, the Connected-mode states currently specified are denoted CELL_PCH, URA_PCH, CELL_FACH and CELL_DCH. 3GPP Technical Specification (TS) 36.331 v8.8.0, Radio Resource Control (RRC), Protocol Specification (Release 8) (December 2009) specifies the RRC protocol for the radio interface between a UE and eNBs in LTE and other communication networks. 3GPP TS 25.331 v8.9.0 specifies the RRC protocol for the radio interface between a UE and BSs and RNCs in UTRANs.
In 3GPP Release 10 (Rel-10) specifications, MDT for a UE in a CONNECTED mode state is based entirely on currently specified radio measurements. For logged MDT, the UE stores (for up to 48 hours) its measurements in one or more measurement logs, and a measurement in an MDT log is linked to a time stamp that is available in the UE. Upon a UE indication transmission to the network (i.e. when the UE is ready), the network may request the UE to report the measurement logs. Based on the report, the NW operator has the opportunity, for example, to identify potential coverage problems in order to reduce costs for network deployment and operation.
One of the requirements for MDT according to 3GPP TR 36.805 is that the MDT measurements, whether immediate or logged, are time-stamped and may also be linked to “available” location information and/or other information or measurements that can be used to derive location information. 3GPP TR 36.805 currently specifies only RSRP measurements for that purpose. Reporting the location information associated with MDT measurements is optional and subject to its availability, which is relevant in particular for the IDLE mode since most positioning methods require the UE to be in a CONNECTED mode.
Thus, a constraint that needs to be accounted for in MDT is that the availability of location information is subject to UE capability and/or UE implementation. In addition, the MDT functionality should avoid long active periods and extensive use of a UE's positioning components, because doing otherwise significantly increases the UE's power consumption, which is undesirable in typically battery-powered devices.
Logged MDT measurements that have been considered so far according to 3GPP TR 36.805 include, as noted above, downlink pilot signal measurements, determinations of whether the UE's serving cell signal level has become worse than a threshold, determinations of whether its transmit power headroom is less than a threshold, and determinations of Paging Channel failure, Broadcast Channel failure, Random Access Channel failure, and radio link failure. All of those measurements may include at least the following information: location information (e.g., the location at which the event and/or measurement took place), time information (e.g., the time at which the event and/or measurement took place), cell identification (e.g., at least the identity (ID) of the serving cell is always included), and radio environment measurements (e.g., cell measurements that are available at the event and/or average cell measurements during a certain period before and/or after the event. The cell measurements typically include RSRP and RSRQ measurements.
If global navigation satellite system (GNSS) location information is “available”, the MDT location information consists of latitude and longitude (which are mandatory), altitude (which is optional, contingent on availability), velocity (which is optional, contingent on availability), and movement direction (which is optional, contingent on availability). GNSS is a generic name for satellite-based positioning systems with global coverage. Examples of GNSS systems include the U.S. Global Positioning System (GPS), the European Galileo, the Russian Glonass, and the Chinese Compass. With GNSS, a position is typically obtained by triangulation based on measurements of times of arrival of satellite signals.
It has not yet been decided when GNSS location information is considered “available” for a logged MDT measurement, e.g., the acceptable time lag between an MDT measurement and determination of the corresponding location information. If GNSS location information is not available, the UE uses radio frequency (RF) fingerprint information, which typically consists of cell IDs and RSRPs for up to six intra-frequency neighbor cells. In addition to the above, an Evolved Cell Global Identifier (ECGI) of the serving cell on which the measurement was taken is always included.
In the logged-MDT case, positioning information updates that trigger MDT measurements are expected to come mainly from standalone UE-based positioning (e.g., GPS location information), and in the immediate-MDT case, positioning information updates that can serve as triggers may come from network control-plane location services (LCS) or user-plane positioning or standalone positioning.
To further enhance the location information, it has been recently proposed to also include location uncertainty information, which may be available with a location result. When the location information follows a standardized format (e.g., when it is received from another network entity, such as a positioning server, over a standardized interface), the availability of location uncertainty information also depends on the format. Some standardized location information formats do not have information elements for location uncertainty, and formats that allow for location uncertainty information typically make the uncertainty information element optional.
Table 1 shows location reporting formats that have been standardized in 3GPP, where a format is associated with a Geographical Area Description (GAD) shape according to 3GPP TS 23.032 v9.0.0, “Universal Geographical Area Description (GAD)” Release 9 (December 2009).
TABLE 1Position reporting formats in 3GPPPositionreportingIncludesIncludesformatDescriptionuncertaintyconfidencePolygonThe polygon format isYesNodescribed by a list of 3-15latitude, longitude corners,encoded in WGS 84 co-ordinates. This format maybe obtained by applicationof cell ID positioning inLTE.Ellipsoid arcThe ellipsoid arc isYesYesdescribed by a center point(eNodeB antenna position),encoded as latitude,longitude in WGS 84 co-ordinates. Furthermore, theformat contains an innerradius of the arc, athickness of the arc as wellas the offset angle(clockwise from north) andthe included angle (openingangle). Together, theseparameters define acircular sector, with athickness and with left andright angles, see [1] fordetails. This format is, e.g.,produced by cell ID + TApositioning in LTE.EllipsoidThe ellipsoid point format isNoNopointdescribed by a center point,encoded as latitude,longitude in WGS 84 co-ordinates.EllipsoidThe ellipsoid point withYesNopoint withuncertainty circle formatuncertaintyconsists of a center point,circleencoded as latitude,longitude in WGS 84 co-ordinates, in combinationwith an encoded radialuncertainty radius.EllipsoidThe ellipsoid point withYesYespoint withuncertainty ellipse formatuncertaintyconsists of a center point,ellipseencoded as latitude,longitude in WGS 84 co-ordinates. The uncertaintyellipse is encoded as asemi-major axis, a semi-minor axis and an anglerelative to north, countedclockwise from the semi-major axis. This format istypically produced byOTDOA and A-GPSpositioning in LTE.EllipsoidThe ellipsoid point withNoNopoint withaltitude format is encodedaltitudeas an ellipsoid point,together with an encodedaltitude.EllipsoidThis is the formatYesYespoint withcommonly received from A-altitude andGPS capable terminals. Ituncertaintyconsists of an ellipsoidellipsoidpoint with altitude and anuncertainty ellipsoid, thelatter encoded with a semi-major axis, a semi-minoraxis, an angle relative tonorth, counted clockwisefrom the semi-major axis,together with anuncertainty altitude. Thisformat is typically producedby A-GPS positioning inLTE.
Two approaches to considering location uncertainty information with reported MDT measurements have been discussed: (1) including the uncertainty information as an element in reports/logs upon availability, and (2) reporting/logging MDT measurements for which (a) the uncertainty satisfies a certain acceptable uncertainty level configured by the network, or (b) the uncertainty exceeds a certain acceptable uncertainty level. Including location uncertainty information in a report or log is preferable, since it provides more flexibility for processing collected MDT measurements.
Signaling of MDT measurements, including associated location information when available, is via RRC signaling. Available MDT measurements performed for intra-frequency/inter-frequency/inter-radio access technology (inter-RAT) can be reported. The set of neighbor cell measurements and associated information that can be reported for MDT currently includes: RSRP and RSRQ for E-UTRAN; RSCP and Ec/No for UTRAN; Rxlev for a GSM and Enhanced Data Rates for GSM Evolution (EDGE) Network (GERAN); carrier frequency (for inter-frequency/inter-RAT); and physical cell ID of the logged cell.
Current approaches to handling location information and location uncertainty information suffer from a number of drawbacks. For example, location information is often erroneously treated as of unsatisfactory quality when it is not. In addition, location information is usually limited to GNSS information, or when GNSS information is not available, to RSRP fingerprints. Moreover, new uses of MDT measurements, e.g., to support network compliance with new or changed official regulations, are limited. Furthermore, UE memory and signaling resources are limited, which can limit MDT measurements or impact the quality. Further, the quality of the reported MDT measurements is currently not known to the network.